With the ever expanding use of increasingly sensitive electronic equipment, the demand for packaging and other materials that protect electronic components from electrostatic discharge has increased dramatically. Electrostatic discharge is generally recognized as one of the most destructive--perhaps THE most destructive--phenomenon confronting the electronics industry. Modern electronic components such as printed circuits and microchips can be damaged by overheating and/or dielectric breakdown from static discharge of as little as 50 volts, and static charges of 10,000 volts or more can be, and frequently are, generated through friction, motion, separation of dissimilar materials and induction in the handling, packaging and shipping of electronic components.
Two general types of protection are commonly employed. The devices may be shielded from electrostatic or radio frequency fields with conductive packaging, incorporating materials such as carbon black or carbon-loaded polymers, providing a "Faraday-cage" effect that bleeds static electricity to ground or causes it to circulate harmlessly until it decays to a level that will not damage the components. This method, however, does not protect the device when the device is removed from the container. Typically, when the device is in the container, not all the leads are in contact with the conductive packaging. Thus, some leads may not be at the same potential as others. Static electricity can build up on the device (through the unprotected leads) from operators, others in the vicinity of the containers or various other causes, and the device could be damaged as soon as the container is opened.
Alternative methods for protecting components from destructive electrostatic discharge include: shorting and/or grounding the leads so as to reduce the build-up of static; dissipating the static with a controlled discharge to ground; and maintaining a uniform electrostatic potential. In some cases, the leads are grounded to discharge the static. In others the leads are simply shorted together so that everything is at the same potential.
Traditional methods of grounding the leads of electrical devices have included: 1) twisting all the leads of the device together, 2) pushing all the leads into a pad of semi-rigid electrostatic discharge (or "ESD") foam material, 3) wrapping the leads in some form of conductive foil, 4) placing the leads into conductive liquid, 5) pushing the leads through a thin piece of conductive foil, and 6) plugging the leads into a specially designed conductive cap. Unfortunately, all of these approaches suffer from serious drawbacks.
With the first method the twisted leads have to be untwisted and then straightened, if possible. This approach adds several costly and time-consuming process steps. In addition, the leads can be weakened by fatigue.
The second method causes unwanted particle materials to be generated during the insertion and removal processes. It can also cause the device leads to be bent, again, causing fatigue and possible fracture. However, the main concern is that this approach cannot be used in clean room areas because of the risk of particulate contamination. Much of the work with electronic components must be done in clean room areas.
The third and fifth methods, wrapping the leads, or pushing the leads through a thin conductive sheet, can bend the leads. As mentioned above, bent leads add process steps, expense, and damage to the leads by fatigue or fracture. Pushing the leads through conductive sheets can also generate particulate contaminates.
Similarly, placing the leads in conductive liquid, the fourth method, is likely to contaminate the leads and require cleaning. Cleaning and inspection add unwanted and costly process steps.
The sixth method of placing the leads into a specially designed conductive cap is expensive. It also adds time, since it requires additional process steps to insert and remove the leads.
The following patents and corporate technical bulletin illustrate a variety of typical protective devices and methods: U.S. Pat. No. 5,232,091 to Hennessey et al, assigned to Eastman Kodak, Co.; U.S. Pat. No. 5,164,880 to Cronin, assigned to Polaroid Corp; U.S. Pat. No. 5,110,669 to Knobel et al, assigned to Dow Chemical Co; U.S. Pat. No. 5,041,319 to Becker et al, assigned to Conductive Containers, Inc; U.S. Pat. No. 5,038,248 to Meyer, assigned to Harris Corp; U.S. Pat. No. 4,553,190 to Mueller, assigned to Minnesota Mining and Manufacturing Co; U.S. Pat. No. 4,333,565 to Woods; and IBM Technical Disclosure Bulletin, Vol.36, No. Apr. 04, 1993.
Hennessey et al, Mueller et al and the IBM Technical Disclosure Bulletin disclose representative devices that shield (and ground at least some of) the leads within conductive containers. The Hennessey container has a conductive strip, inside the container, which is urged against the pins of components packaged in the container when the container is closed. Mueller et al discloses a container for longitudinally receiving components that are sensitive to electrostatic discharge, featuring a rigid longitudinal channel having a transparent conductive layer. The IBM Technical Disclosure Bulletin discloses a somewhat similar packaging device using a module tube constructed of conductive plastic.
Although these packaging containers protect the leads of the devices from unwanted electrical charges while the devices are within the containers or tubes, they do not protect the devices while they are being handled during the various testing or manufacturing process steps. Between each testing or manufacturing test station, the devices must be reloaded back into their protective containers or tubes. Then they must be unloaded again and inserted into the next tester station. At each loading and unloading stage, as well as in the test position within the testers themselves, the devices are exposed to possible damage from unwanted electrical charges.
Cronin, Knobel et al, Becker et al, Meyer, and Woods all disclose protective materials, methods and apparatus that rely primarily on the controlled shorting and/or discharge of static electricity. Cronin discloses an apparatus for protecting printed circuit (or "PC") boards with conductive pads along at least one edge for connecting the board with an edge connector. Cronin's system includes a dual-hinged structure that urges electrically conductive grounding bars against the conductive pads, shorting them together. When the board is inserted into an edge connector, the connector pushes on the hinged structures and moves the electrically conductive grounding bars away from the conductive pads. The pads are then free to engage the edge connector. This structure seems well-suited for the relatively robust contacts on the edge of PC boards, but much less adaptable to the fragile wire leads of other typical electronic devices such as microchips, which can be damaged by fatigue and fracture.
Knobel et al and Becker et al disclose two different prior art grounding materials. Knobel et al discloses a laminated plastic material useful in making shipping bags for protecting electronic devices from build up of static charges on their leads during transport and storage. Becker et al discloses a multilayered foam material for protecting leaded electronic devices from built-up electrostatic charges through lead insertion into a foam. Pushing the leads through either of these materials (if even possible for the Knobel shipping bag material) can bend the leads, adding unwanted process steps to straighten the leads, expense and fatigue damage to the leads.
Meyer discloses an electrostatic discharge protection device that includes shorting elements for shorting the leads together and an actuator, responsive to being inserted into a tester, to disengage the shorting elements from the leads. However, no means is provided for compensating for different lead lengths or diameters.
Woods discloses a package with an electrically conductive pad in which microcircuit devices may be embedded for storage, transportation and field use. The package, which has a top panel that covers the embedded components, is said to inhibit electrostatic charge buildup, but it would appear that the primary protective effect is through short-circuiting the leads of the components. In addition, the system only functions as a protective carrier for transporting the devices to the testing line. Once there, the devices are removed from their protective packages and left unprotected for the entire testing time.
To offer the maximum protection to the devices to be tested at the minimum test cycle time and cost, it is necessary to reduce the handling time of the devices to one loading/unloading cycle in the test fixtures and to protect the devices within the test fixtures before and after the actual tests on the devices.